Apparatus for Mixing a Liquid Medium Into a Gaseous Medium

ABSTRACT

An apparatus for mixing liquid medium into a gaseous medium flowing through an exhaust pipe from an internal combustion engine is provided. The liquid medium is supplied to the exhaust pipe with varied mass flow via an injector nozzle directed into the exhaust pipe. The exhaust pipe has a mixer placed downstream of the injector nozzle and a flow path disposed parallel with the mixer and extending through a perforated surface.

TECHNICAL FIELD

The present invention relates to an apparatus for mixing a liquid medium into a gaseous medium flowing through an exhaust pipe from an internal combustion engine, which liquid medium is supplied to the exhaust pipe with varied mass flow via an injector nozzle directed into the exhaust pipe.

BACKGROUND

In devices for reducing Nox emissions in diesel engines, it is known to use mixers for mixing liquid, for example a reducing agent such as urea, into the exhaust gases of the engine so as, through vaporization and mixing, to produce a more homogenous gas stream to a SCR catalyzer placed downstream of the mixer. One problem with diesel engines in heavy-duty vehicles is that the supply of reducing agent must be accurately matched to the varying operating conditions of the engine, i.e. principally to the variations in the exhaust flow.

Under maximum supply of reducing agent, the vaporization/mixing-in may sometimes fail to work perfectly, in which case liquid in droplet form can reach and move downstream along the walls of the exhaust pipe until vaporization takes place. A recognized solution to the problem is to fit a plurality of turbulence-creating devices in the exhaust pipe, e.g. in the form of perforated plates or wings which produce a deflection of the gas flow, thereby increasing the flow time and hence the vaporization. Unfortunately, this gives rise to an increased fall in pressure in the exhaust line, which adversely affects the breathing of the engine.

ACCOUNT OF THE INVENTION

One object of the invention is therefore to produce a mixing apparatus which is simple and effective with low fall in pressure and good mixing capability.

To this end, the apparatus according to the invention is characterized in that the exhaust pipe has a mixer placed downstream of the injector nozzle and a flow path disposed parallel thereto and extending through a perforated surface. This configuration of the mixing apparatus allows a smaller mass flow, associated with low rev speeds, to pass freely in the exhaust pipe past the mixer. As the mass flows increase, the mixer will gradually force a larger share of the flow through the parallel flow path, which increases the vaporization and additionally produces the parallel flow comprising turbulence and mixing of the exhaust gases when it is reunited with the main flow.

Advantageous variants of the invention emerge from the following subclaims.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in greater detail below, with reference to illustrative embodiments shown in the appended drawings, in which:

FIG. 1 illustrates the placement of an injection nozzle on an exhaust pipe, and

FIG. 2 shows in broken view a longitudinal section through an apparatus according to the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a first part-portion 10 of an exhaust pipe connected to an internal combustion engine, which part-portion is provided with an injector 11 for injecting liquid into the exhaust stream. The liquid can be constituted, for example, by urea or fuel. The pipe expediently has a circular cross section and is delimited against the environment by means of an outer wall 12. Arrows 13 illustrate the exhaust flow in the pipe. The injector 11 has a spray direction illustrated by the dash-dot line 14, but as a result of the exhaust flow directed from left to right in FIG. 1 the liquid droplets will be sprayed into the exhaust pipe in the form of a cone, as illustrated by the lines 14 a. The internal combustion engine can be constituted, for example, by a diesel engine operating at varied speed for the propulsion of a heavy-duty vehicle.

A second part-portion 15 of the exhaust pipe is shown in FIG. 2 and is situated downstream of the first part-portion 10 comprising the injector 11. Placed downstream of the nozzle is a mixer, which, as shown in FIG. 2, can be constituted by a flat plate 16 mounted transversely to the flow direction, or by a mixer suitably configured in a known manner. The plate 16 is shown with perforations 17 in the vicinity of the pipe wall 12. A principal purpose of the mixer is to create mixing in the exhaust pipe. This creates a fall in pressure, which is exploited in the apparatus according to the invention.

A widened pipe portion 18 is disposed in the region of the plate 16, and the pipe wall 10 is provided with perforations 19 into the widened portion 18, upstream and downstream of the plate 16. The pipe portion 18 hence forms a parallel flow path extending past the plate 16. Low exhaust flows can pass freely past the plate 16 with intended mixing-in/vaporization of the injected liquid. When the speed of the internal combustion engine increases, the flow through the exhaust pipe also increases, whereupon the quantity of liquid supplied via the injector 11 is adjusted to the increasing gas flow.

When the internal combustion engine is operating in its upper load range, the plate 16 creates a large flow resistance, whereupon a part of the exhaust flow will travel past the plate via the parallel flow path formed by the pipe portion 18. Liquid droplets which have not had time to vaporize will follow the pipe wall 10 and pass through the perforations 19, whereby the liquid droplets are given increased opportunity to vaporize. Moreover, an effective mixing of the exhaust flow takes place downstream of the plate 16 when the parallel flow is reunited with the main flow.

It has proved advantageous to configure the perforations 19 upstream of the plate 16 such that they together have an area which is between about 0.5-3.0 times the cross-sectional area of the exhaust pipe. On the downstream side of the plate 16, the perforations 19 can have a combined area which is either the same size as or is different than the area upstream of the plate. These area ratios have been illustrated in FIG. 2 by means of the measurement arrows a and b.

The invention should not be deemed limited to the illustrative embodiments described above, but rather a number of further variants and modifications are conceivable within the scope of the following patent claims. 

1. An apparatus for mixing liquid medium into a gaseous medium flowing through an exhaust pipe from an internal combustion engine, the apparatus comprising the exhaust pipe and an injector nozzle, the liquid medium being supplied to the exhaust pipe with varied mass flow via the injector nozzle directed into the exhaust pipe, wherein the exhaust pipe has a mixer placed downstream of the injector nozzle and a flow path comprising a widening of the exhaust pipe and disposed parallel with the mixer and extending through a perforated surface, wherein the perforated surface comprises a perforated portion of a wall of the exhaust pipe.
 2. The apparatus as claimed in claim 1, wherein perforations of the perforated part upstream of the mixer have an area which is between about 0.5-3.0 times a cross-sectional area of the exhaust pipe.
 3. The apparatus as claimed in claim 2, wherein the mixer comprises a plate in the exhaust pipe.
 4. The apparatus as claimed in claim 1, wherein the mixer comprises a plate in the exhaust pipe. 